1. Field of the Invention
The present invention relates to transmitting stages and in particular to transmitting stages including an amplitude locked loop and a phase locked loop in order to amplify an amplitude- and phase-modulated signal via a non-linear power amplifier and, if applicable, transmit the same via a wireless transmission channel.
2. Description of the Related Art
For mobile communication services only a limited number of frequency bands exists. The required channel bandwidth in data transmission and the possible data rate are decisive factors characterizing the effectiveness of a transmission system. Thus, within a frequency band, a maximum data rate is aimed at. Different methods exist which facilitate allowing a higher data rate at the same channel bandwidth and thus enable a more efficient message flow.
In recent years, in the field of mobile communication the GSM (global system for mobile communication) standard became established. Here, a GSMK (Gaussian minimum shift keying) modulation is used. The GMSK counts among the so called CPM (continuous phase modulation) modulation methods. This is a non-linear digital modulation method having a constant amplitude and a continuous phase.
The increase of the message flow may be effected by a change of the modulation method. Here, instead of a GMSK modulation a 3π/8-8PSK (phase shift keying) modulation for the GSM EDGE (enhancement data ratio for GSM evaluation) standard or a QPSK (quadrature phase shift keying) modulation for the UMTS (universal mobile telecommunication system) standard is used. The 3π/8-8PSK modulation and the QPSK modulation contain also an amplitude component apart from the phase modulation. By this, a transmission of additional information for increasing the data rate is possible with the same channel bandwidth.
One critical point in the mobile terminal device is the transmission performance of the RF transmission amplifier with regard to the RF signal to be transmitted for the EDGE and UMTS standard. In contrast to the GMSK modulation, in the 3π/8-8PSK modulation and the QPSK modulation the phase and the amplitude are modulated. The result is a spectral widening of the output signal after the non-linear power amplifier or a clear distortion of the transmission signal, respectively. This leads to an increase of the bit error rate (BER) with the same reception field strength.
In order to minimize these distortions, actually the use of a linear power amplifier would be required. The efficiency of linear amplifiers is, however, with approximately 35% clearly worse as compared to non-linear power amplifiers achieving an efficiency of over 50% up to 60%.
The high energy consumption of the system due to the low efficiency of the components stands in contrast to the aim to achieve operation times of the mobile station which are as long as possible.
Signal reconstruction techniques like polar loop enable the use of non-linear power amplifiers also for the EDGE standard and the UMTS standard.
So-called polar loop transmission circuits are for example described in the U.S. Pat. No. 4,481,672, WO 02/47249 A2, the U.S. Pat. No. 4,630,315 or GB 2368214 A.
Also EP 1211801 A2 discloses a polar loop transmission circuit suitable for future mobile radio systems comprising a phase and amplitude modulation and which may also be used for prior systems according to the GSM standard. The polar loop transmission circuit includes a power amplifier receiving a signal from a VCO on the input side. The control signal for the VCO is gained by amplitude limitation of the transmission signal as the nominal signal and by an amplitude limitation of an actual signal, a subsequent phase comparison of the limited signals and a following low-pass filtering.
The amplitude control signal for the controllable non-linear power amplifier is generated by rectifying the transmission signal as the nominal signal, rectifying the actual signal, a subsequent differential formation by means of a differential amplifier and a subsequent low-pass filtering.
The actual signal is branched for the amplitude regulation and for the phase regulation from the output of the non-linear power amplifier, fed to a programmable amplifier and then mixed down to an intermediate frequency, supplied to a ramp-controllable amplifier and then on the one hand fed into the rectifier for an amplitude regulation an on the other hand into the limiter for a phase regulation.
At the control terminal of the programmable amplifier into which a feedback signal branched from the output is primarily fed, using a control signal the power level at the output of the polar loop transmission circuit may be regulated. Hereby, the programmable amplifier is a linear amplifier linearly attenuating the signal to be supplied at its input. The voltage of the high-frequency signal provided at its output, however, is not linearly dependent on an adjustment signal to be supplied at the control terminal and is e.g. 2 dB per least significant bit change of the adjustment signal.
Typical polar loop transmission circuits, as they are disclosed in EP 1211801 A2, are suitable for cellular radio telephones according to the GSM standard and for alternative modulation methods in which a phase and an amplitude modulation have to take place.
Such cellular mobile radio systems have, as further components, an automatic amplification regulation in so far that in the base station and/or in the mobile part a field strength measurement is performed in order to then upregulate the transmission power of the mobile telephone and/or the base station when it is determined based on a low reception field strength that the currently present transmission channel is not satisfying. As the current consumption is only critical for the mobile parts, often only a transmission power regulation is performed in the mobile part.
So it is desirable on the one hand in the interest of a low bit error rate to use a very high transmission power as thus automatically the signal/noise ratio at the receiver and thus the bit error rate decreases. On the other hand, a high transmission power is not desired due to the increasing resistance from the population. Further, a high transmission power leads to the fact that the cells may only be construed in a relatively coarse-meshed way or that a carrier frequency may not be “re-used” in a cell raster as often as possible, respectively, in order to enable a high number of subscribers in the limited frequency band.
A high transmission power comprises the problem, in particular when using non-linear amplifiers, that side channel interferences may increase, i.e. that a transmitter actually specified for a carrier frequency also sends out power, due to its non-linearity, into a side channel in which it should actually transmit nothing or only below a threshold. Such a transmission device is not in accordance with the regulation when the so-called side channel emission is above a certain specification. So it is for example required for the mentioned EDGE standard that the spectrum of the output signal of the radio device is at a file frequency of +/−200 kHz with regard to a carrier frequency smaller than −54 dBc, and further at an offset frequency of +/−300 kHz with regard to the carrier below −60 dBc.
It is required for the UMTS standard that the spectrum of the output signal is better than −45 dBc in the whole neighboring channel.
All of those requirements seem to indicate that the signal power of the mobile telephone has to be as low as possible, i.e. so small that a required bottom bit error rate is just about fulfilled.
In addition, for mobile telephones the special requirement exists that they have to be cheap, as the mobile telephone market has developed to be an extremely competitive market in which already small savings enabled a system to be capable of surviving, i.e. that the same was accepted by the market, while the other system was not accepted in the market.
For mobile telephones, thus a transmission power regulation is used which is as sensitive as possible, which in case of a good transmission channel reduces the transmission power as fast and as far as possible, which, however, is also able, in case of a mainly only prevalent bad channel, to increase the transmission power very fast and in particular very strongly. A polar loop transmission circuit thus has to work in a very high dynamic range of the power amplifier on the one hand and on the other hand handle a very high dynamic range with regard to the amplitude locked loop and the phase locked loop, which together form the polar loop.
Disadvantages of the concept disclosed in EP1211801 A2 are that the setting of the output power is performed via the programming of the programmable amplifier in the feedback branch. The programmable and the downstream ramp-operated amplifier thus have to provide an output signal having a high dynamic which in one case, i.e. with maximum output transmission power, is very small, and which in the other case, i.e. with a minimum output power, is very high and thus in particular gets close to the amplitude of the output signal of the transmission signal generator.
It was found out that a signal having a too high dynamic in the feedback path leads to the fact that the rectifier detector for determining the amplitude actual signal has to be implemented very expensively. Even more serious is the fact that the amplitude limiter required for generating the phase actual signal also has to be implemented very expensively, as, when the input signal becomes very small within the same, it requires an extremely high amplification in order to achieve an amplitude limitation in order to provide a phase actual signal. This has to be guaranteed in any case, however, because as soon as the amplitude limiter for the generation of the phase actual signal provides no defined output signal any more, the phase locked loop of the polar loop comes out of lock, which leads to a complete loss of the data transmission. In other words, a failure of the phase locked loop of the polar loop leads to the fact that the message connection is immediately interrupted and that it has to be waited up to a new synchronization, which is cumbersome and not tolerable, as it may easily be seen.